Comment 5 for bug 1077912

1) No, the sparsity pattern _shows_ what I mention. There is only one issue here, and my bug title is wrong because assemble_system is not that issue :)

2) As a minimum the documentation should be changed to reflect the actual behaviour. However I find the current behaviour counterintuitive, while e.g. adding another method string which allows internal facets explicitly would make it clear what happens.

This code should define _boundary_ conditions everywhere:
boundaries = MeshFunction("uint", mesh, dim-1)
boundaries.set_all(7)
bc = DirichletBC(V, mesh, boundaries, 7)

while this could be syntax for allowing interior facets with the topological method:
bc = DirichletBC(V, mesh, boundaries, 7, method="interior")

still misuse of the term "BC", but at least with clear intention.